1. Field of the Invention:
The present invention relates to a shift control device for an automatic transmission for automobiles where an electric control system is used and, more particularly, a shift control device for an automatic transmission whereby good timing of hydraulic servo operations during shifting is obtained, and undesirable shocks during shifting is prevented.
2. Description of the Prior Art:
In an automatic transmission with a torque converter, the hydraulic pressure is adjusted in accordance with each gear stage by installing a variety of valves on a hydraulic circuit while the hydraulic pressure is also modulated in accordance with the car speed and throttle opening to control the shock during shifts. An accumulator is also provided for the purpose of adjusting the oil pressure during the transition stage from the start to the completion of the shift. However, since the pressure schedule controlled by the accumulator is limited, the optimum hydraulic adjustment is not obtained to prevent shocks throughout the operating range. Further, a controller equipped with a number of accumulators and valves makes the hydraulic circuit complicated. On this account, as disclosed in U.S. Pat. No. 4,031,782 and the like, a controller for an automatic transmission equipped with an electronic control system as a shock control device has been provided. Said shock control device simplifies the hydraulic circuit by detecting electrically car speed, change in the torque of an output shaft and so on, and performs hydraulic adjustment by opening and closing of a solenoid valve installed at the appointed location of a hydraulic circuit. However, in the controller for an automatic transmission with said shock control device disclosed in the prior art, no consideration or countermeasure has been provided in case of a malfunction generated in the hydraulic circuit due to valve sticking and so on. In addition to this, the controller of the prior art was made complicated by the use of multiple solenoid valves for adjusting oil pressure as required during the N-D shift performed manually, and during the 1-2 shift, 2-3 shift, and 3-4 shift performed automatically, for example, in a 4-speed automatic transmission. A timing and oil pressure control were further required to control proper timing for a clutch-to-clutch shift without the invervention of a one way clutch in order to prevent undesirable shift shock. The timing valve was, however, insufficient in this timing function for shock prevention especially at low-speed shift or power-off shift.